Thermoformable laminated structures

ABSTRACT

A thermoformable essentially rigid structural sandwich laminate of thermoplastic polymeric material having a cellular core member and a pair of exterior non-cellular skin members of thermoplastic material of normal density adhesively united to the surfaces of the core member, wherein each of the elements which form the skin and core members respectively maintain their structural integrity within the laminate in subsequent thermoforming. The cellular core member may be either a single element or may be formed of a plurality of elements or laminae each of which may be of the same or of different densities. A process of preparing the thermoformable laminate is provided which comprises adhesively uniting and adhering to the outer planar surfaces of the cellular core member, sheets of non-cellular thermoplastic material and includes the steps of adhesively uniting the several elements or laminae that make up a core member of a composite construction. The invention also relates to a process for the manufacture of relatively rigid formed items from said laminate by thermoforming of the aforesaid laminate by the application of heat under controlled conditions followed by the application of differential pressure to said laminate, the heat being applied through the exterior non-cellular thermoplastic surfaces of the laminate to modify the thickness, density and cell structure of the cellular core member under controlled thermoforming conditions.

United States Patent 11 1 Massey et al.

1 THERMOFORMABLE LAMINATED STRUCTURES [75] Inventors: David 11. Massey,Glencoe, 111.;

Robert B. Anderson, Edina, Minn.

[73] Assignee: Xox Corporation, Lombard, 111.

[22] Filed: Mar. 9, 1970 [21] Appl. No.: 17,573

Related US. Application Data [63] Continuation-in-part of Ser. No.796,738, Feb. 5,

1969, abandoned.

[52] US. Cl 161/161, 1 6l/l90, 161/254, 161/253, 161/120, 156/210, 9/6[51] Int. Cl B32b 5/18, B32b 3/00 [58] Field of Search 161/159, 160,161,161/190, 254; 9/6; 156/210 [56] References Cited UNITED STATES PATENTS2,586,275 2/1952 Toulmin 161/159 3,041,220 6/1962 Martin et a1. 161/2543,070,475 12/1962 Carlson et al.... 161/161 3,070,817 l/l963 Kohrn eta1. 161/254 3,174,166 3/1965 Ehrenberg et al. .1 9/6 3,317,363 5/1967Weber 156/210 3,429,085 2/1969 Stillman 161/160 3,180,778 4/1965Rinderspacher 156/311 3,332,646 7/1967 Kellett et al. 161/160 3,432,3803/1969 Weber 161/161 3,503,841 3/1970 Sterrett 161/161 PrimaryExaminer-Morris Sussman Attorney-Pendleton, Neuman, Williams & Anderson51 Aug. 28, 1973 [5 7] ABSTRACT A thermoformable essentially rigidstructural sandwich laminate of thermoplastic polymeric material havinga cellular core member and a pair of exterior noncellular skin membersof thermoplastic material of normal density adhesively united to thesurfaces of the core member, wherein each of the elements which form theskin and core members respectively maintain their structural integritywithin the laminate in subsequent thermoforming. The cellular coremember may be either a single element or may be formed of a plural ityof elements or laminae each of which may be of the same or of differentdensities. A process of preparing the thermoformable laminate isprovided which comprises adhesively uniting and adhering to the outerplanar surfaces of the cellular core member, sheets of noncellularthermoplastic material and includes the steps of adhesively uniting theseveral elements or laminae that make up a core member of a compositeconstruction.

The invention also relates to a process for the manufacture ofrelatively rigid formed items from said laminate by thermoforming of theaforesaid laminate by the application of heat under controlledconditions followed by the application of differential pressure to saidlaminate, the heat being applied through the exterior non-cellularthermoplastic surfaces of the laminate to modify the thickness, densityand cell structure of the cellular core member under controlledthermoforming conditions.

8 Claims, 6 Drawing Figures Patented Aug. 28, 1973 2 Sheets-Sheet 2mmQOU 0 o o o o o QK V oo o o 0 0 M o O 0 a O 0 on o 00 0 0 a xoxxvxOxokkxk INVENTORS DAV/D H. MASSEY ROBERT B. A NDERSON M Q/M ATTYS r lTHERMOFORMABLE LAMINATED STRUCTURES Cross Reference to RelatedApplication This application is a continuation-in-part of U.S. Ser. No.796,738 filed February 5, 1969 now abondoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to rigid thermoformable structural laminates in sheet form andto methods of preparing them. The laminates include non-cellular skinadhesively united to a cellular core member which may be a singlecellular element or may be formed from a plurality of thermoplasticcellular sheets or elements adhesively united together. The laminatedoes not involve the use of chemical blowing agent or vulcanizingagents, and may be prepared without the application of heat and pressureto form the cellular core member.

The invention also relates to a process of thermoforming the laminatedescribed above and the formation of finished goods thereby. Theproducts so formed are characterized by low cost and improved strengthin selected areas without using in situ foaming.

2. Description of the Prior Art The formation of cellular laminates andstructures by a variety of techniques is well known. Most of the rigidcellular laminates have been prepared by the in situ formation ofcellular cores sandwiched between cover sheets or encasements in a mold.Chemical blowing agents or gas forming agents mixed as an integral partof the core element mix are activated by heat or otherwise in the mold.This activation or generation of gas produces a typical closed cellstructure in the core element which expands to fill the configuration ofthe mold. The cover sheets which form the outer surface of the laminateare, of course, held in position by foaming molds and the foam expandsto conform to the contours of the preformed skin. These procedures,however, are both expensive and difficult to control. Quite frequentlythe core element contains both blowing agents and curing agents whichrequire extended curing cycles.

Pat. No. 3,070,817 patented Jan. 1, 1963 describes the formation oflaminated elastomeric materials and the process of manufacturing aplastic therefrom. In this process a laminate is described which isformed from a plurality of laminae of elastomeric, vulcanizable rubberysheets, the innermost of which contains a chemical blowing agent. Thelaminate is formed under heat and pressure resulting in a bonding of thelaminae together by virtue of the vulcanizing agents contained therein.Due to the heat in the laminating process, the chemical blowing agentdecomposes and forms a cellular core element wherein contained. Thepressure of the laminating apparatus (200 psi.) is such as to preventexpansion of the cellular core element during heating but on release ofpressure it expands to a foam of about pounds per cubic foot density. Inthis vulcanized laminate, the elements are rigidly vulcanized to eachother (unified) and are not free to move laterally with respect to eachother during subsequent thermoforming operations.

It will be noted that the nature of the process is such that structuralintegrity of the layers is lost during the heating or curing processsince the vulcanization, in effect, cross-links the various elements ofthe lay-up into a single unified structure. In the final product, theresuit is an integrated unified laminate wherein the structuralintegrity of each of the components is lost and no longer identifiableas such. The fabrication of this structure obviously presents certainproblems inas- 5 much as the cellular character of the core material isThe present invention provides a low cost process for 5 the manufactureof essentially rigid thermofonnable laminatesin sheet form having acellular plastic foam core without requiring the in situ formation offoam. The laminates of thisinvention are characterized in that theindividual laminae maintain their structural integrity in the formationof the therrnoformable laminate and in subsequent thermoformingoperations to produce finished goods. The laminates are furthercharacterized in that the laminae are free to move laterally withrespect to each other while maintaining, however, the integrity of thelaminate during molding or thermoforming operations. The laminates areprepared without the use of vulcanizing or cross-linking agents and haveunexpectedly high physical properties especially in the area of impactresistance and flexural strength.

SUMMARY OF THE INVENTION In the most preferred form the cellularelements in a multilayer or multi-component core member are of differentdensities. The present invention also relates to a process of formingthe rigid therrnoformable structural laminate which comprises adhesivelyuniting a pair of exterior skin sheets of non-cellular thermoplasticmaterial of normal density to a cellular thermoplastic core member ofthe types described above.

The invention further relates to a process for the manufacture ofrelatively rigid formed items from the thermoformable laminate bythermoforming of said laminate.

The cellular core member is preferably formed of a thermoplasticmaterial, such as, extruded oriented polystyrene, polyethylene orcopolymers of styrene, acrylonitrile, butadiene, cross-linkedpolypropylene, or the like, and has a density of from about 3 to about30 pounds per cubic foot, preferably from 4 to 20 pounds per cubic foot.The core and skin members may be characterized as thermoformable.

In the most preferred form of the present invention, the core member orlay-up is formed by adhesively uniting together a plurality of foamedthermoplastic plurality of laminae or plies adhesively united together.

sheets or elements. In one preferred form the innermost cellular coremembers have a density that is less than the outer cellular core membersalthough both are cellular thermoplastic foams. Thus in one form theinnermost core elements have densities of from 3 to about pounds percubic foot, and the other or outermost cellular core members havedensities of from 12 to about 30 pounds per cubic foot, preferably 12 topounds per cubic foot.

The outer or exterior sheets of the laminate forming the outer skinthereof, are formed from non-cellular relatively rigid thermoplasticsheeting of normal density, such as for example, rigid vinyl orpolyvinyl chloride sheeting, rigid acrylic sheeting, or rigidacrylonitrile, butadiene, styrene (ABS) sheeting, other polystyrenecopolymers, polycarbonates, and acrylatemethacrylate copolymers having adensity of from about 60 to 90 pounds per cubic foot and from 5 to 150mils in thickness.

In one preferred embodiment the cellular core members contain someresidual blowing agent which remains after the initial formation of thecellular sheet. Initially the sheets are formed using from 3 to 6percent blowing agent which is capable of volatilization at extensivetemperatures.

The blowing agent is preferably a non-chemical type, that is to say, onewhich is of the inert volatile organic fluid type which is converted toa gas at thermoforming conditions and does not rely on chemicaldecomposition or interaction to form a gas. When a volatile organicfluid is used there is some substantial residual solvent remainingtrapped within the cells. Due to diffusion the residual volatile organicfluid remaining within the cells slowly escapes from the extruded foamstructure at a rate of approximately 0.005 pounds per 100 pounds of foamper day. Ordinarily when the extruded foam stock is relatively freshthere is no necessity to take this factor into account but it should,however, be understood that after a substantial period of time, i.e., 30days or more, the residual volatile organic fluid within the cells hasbeen sufficiently depleted so that very little residual cell expandingcapability attributable to the fluid per se is left. The preferredblowing agent which are used in forming the cellular structures aregases at normal atmospheric pressure. Those thermoplastic cellularmaterials of very low density have a higher capacity for expansion uponheating to the softening stage by virtue of the fact that they contain agreater percentage of trapped gas, air, or volatile organic fluid withinthe thermoplastic matrix. In those instances where the cellular materialcontains some residual organic fluid blowing agent additional expansionand concomitant reduction of density on heating is possible by virtue ofthe fact that the blowing agent is converted from the liquid state tothe gaseous state.

It should also be recognized that in any thermoforming processes whichnecessarily involve a further heating of the laminate of the presentinvention, excessive heating or heating under uncontrolled pressureconditions will result in over-expansion of the foam cells, rupture ofthe cell walls, followed by the collapse of the cells to produce aproduct of higher density and when taken to the logical extreme willresult in a product which reaches the limiting value of the density ofthe polymeric thermoplastic material in its normal, noncellular state.

It has also been found that in the heating of the multielement corelaminates of this invention in the thermoforming operation where theheat is principally in the radiant form is transferred from or throughthe outer surfaces, there is a tendency for the composite cellular coreand exterior non-cellular skin laminae to progressively rise intemperature so that the outermost surfaces attain the highertemperatures and the innermost core elements or laminae areprogressively lower in temperature. Even with this lower temperature theless dense innermost cellular elements are the most responsive totemperature and therefore expand at a higher rate, whereas the outermostcellular elements may, depending upon the temperature and pressureconditions of forming, decrease in density at a lower rate.

The organic fluid blowing agents used in forming the laminae of thepresent invention are pentane or similar C-S hydrocarbons and materialswhich have a similar volatilization temperature range, such as, Freon112 and Freon 114 or mixtures of any of these materials. The extrudedfoam products used in forming the core members in the preferred aspectsof the present invention are of the closed cell type and range inthickness from about 30 to mils (0.030 to 0.150 inch). They have cellsizes averaging from about l/64 inch at the high end of the range downto l/3,000 inch at the smaller end of the range.

It should, however, be understood that cellular material which containsno blowing agent has the capacity upon heating of substantial furtherexpansion by virtue of the entrapped air. While reference is made inthis context to expansion of the cellular foam material, it should berecognized that under particular forming techniques an increase indensity in selected areas of a formed part can be achieved. Thisprovides certain ad ditional puncture resistance and compressivestrength properties in these selected areas. In those selected areaswhere there is an increase in density of the cellular component, thecellular character is sharply reduced.

Thus in a molding or thermoforming situation a laminate as a whole,prepared in accordance with the present invention, is heated from theexterior skin surfaces and has a tendency to expand. The mold or formingstructure disposed around the heated laminate will have a tendency tocompress some sections of the laminate and in other areas permit thelaminate to expand to conform to the walls or surface of the moldingform. The ultimate result is a final molded product which, althoughgenerally sheet-like in character, may in some instances, containthinner portions and in some cases thicker portions. The terms thinner"and thicker as used herein are meant to refer to the norm of the generalthickness of the sheet as it is processed. In the normal planar areawhich represents the major proportion of the formed sheet, there may bea material density gradient in the core member from adjacent theexterior skin surfaces to the innermost core elements. The least denseportion will ordinarily be the most interior of the cellular coreelements. The core elements that are disposed between the most interiorof the core elements and the exterior skin surfaces will generally be ofsomewhat higher density and the outer non-cellular skin or exteriorlaminae elements will be, of course, of normal density since they werenon-cellular in the first instance. In those portions of the laminatewhich are of reduced thickness by virtue of the configuration of themold, a similar density gradient pattern may exist in the core. However,the core portion of the laminate that is thin in cross-section willnormally be higher in density on an overall basis than the adjacentportions of the laminate which are of normal thickness.

Conversely in those portions of the laminate where the moldconformations are such as to permit the laminate to expand above theaverage thickness dimension of the thermoformed sheet, the sheet willhave overall reduced densities in the immediate area of the expandedportion. It should be understood that all the elements of the laminateof the present invention maintain their structural integrity duringthermoforming notwithstanding the process or mold that is utilized inthe formation of the product.

The adhesive which is used in uniting the various elements of thelaminate is preferably an elastomeric, rubbery contact type adhesive,i.e., one which is self adhering. Preferably the adhesive is solventbased although aqueous base or emulsion type adhesives may be used. Theelastomeric material used in the adhesive should be compatible with thefoam cores and skin materials. Neoprene or polychloroprenes having amolecular weight in the area of 5,000 are preferred. The resin used inthe adhesive is preferably of the terpene phenolic or butylated phenolictype. When solvent based, the solvents are preferably predominantlyaliphatic and contain some ketones such as acetone and methylethylketone. It should be understood that the solvent formulation should besuch as will not attack the foam of the core sheet causing collapse. Thenature of the adhesive is such that when dry it possesses enoughplasticity to soften and flow when heated in the thermoforming step topermit the plies or laminae to shift or slip past each other asespecially may be noted in the areas of the final product where a curvedor thick section is encountered. The solids content in the solvent basedadhesive is preferably 22 to 24 percent.

BRIEF DESCRIPTION OF THE DRAWING For a more complete understanding ofthe present invention, reference is made to the drawings, wherein:

FIG. 1 is a schematic illustration of a method for forming the laminateof the present invention;

FIG. 2 is a representative enlarged fragmentary cross-section of alaminate formed from the process utilizing the system of FIG. 1; and

FIG. 3 is an enlarged cross-sectional view of a portion of a structureformed through a thermoforming process utilizing the laminate of thepresent invention.

FIGS. 4, 5 & 6 are fragmentary cross-sectional views of otherembodiments of laminates of the present invention.

In accordance with the accompanying drawings the schematic illustrationof FIG. 1 shows a laminating method or process which may be employed toproduce the laminate of applicants invention. In the illustrated processa pair of outer skins or surface laminate in sheet form are providedfrom supply rolls 1 and 2 which layers are sheets of non-cellularrelatively rigid polymeric material of normal density designated 11 and12. Four rolls of cellular core material, 3, 4, 5 and 6, are provided tosupply the core material laminae designated l3, l4, l5 and 16, to besandwiched between the exterior skins. The rolls designated 3 and 4supply extruded cellular thermoplastic material of predetermined densityin sheet form (13 and 14) such as polystyrene. Interior rolls 5 and 6also provide cellular thermoplastic core elements (1.5 and 16) which mayalso be cellular polystyrene in sheet form of predetermined density. Thedensity of the sheets 15 and 16 may be lower than that of sheets 13 and14. The laminae 11 through 16 from rolls 1 to 6 are delivered into acommon laminating zone to provide lamination into the sheet material orlaminate which is designated 18.

As illustrated in FIG. 1, the sheet 11 through 16 are passed through anadhesive spraying section for the application of a layer of adhesivethereto. This adhesive is a self-adhering pressure contact type in acarrier such as a solvent. Adhesive is applied to the planar surfaces ofthe laminae with the exception, of course, of the outer surface of skinmembers 11 and 16. Solvent or carrier in the adhesive is removed byevaporation by passing the coated sheets through a drying section. Afterdrying, the sheets are joined and pressure rolls 19 and 20 are employedto contact the self-adhering adhesive surfaces together with pressure tobond the sheets and form the laminate. The adhesive applied to thesurfaces of the several laminae 11 through 16 forms a mechanical bondtherebetween to adhere these surfaces together.

The lamination operation is also illustrated in FIG. 1 and the formedlaminate may either be cut to desired lengths and stored or may be cutand immediately formed in a thermoforming process.

A prime prerequisite of this particular pressure sensitive contactingadhesive is that the adhesive remains sufficiently flexible through thethermoforming temperature ranges and that the adhesive will form andprovide a proper bond at all temperature ranges while not adverselyaffecting the particular characteristics of the material of theindividual layers as they are subjected to the various loads applied tothe formed or unformed panels.

In the form shown the material forming the individual laminates may beof similar or dissimilar materials but it has been found that the corematerial laminae designated 13, 14, 15 and 16, should be and shouldinclude what are known in the art as expandable substances. Thesesubstances may include expandable polystyrene, expandable ABS or thelike, with the primary requisite being that the material has gonethrough a controlled expansion and extruding process whereby a sheet ofpredetermined density has been provided and which may be furtherexpanded upon the application of heat thereto.

As illustrated in FIG. 1, a plurality of core layers or laminae includetwo layers of cellular thermoplastic material 13 and .14, adjacent theexterior skin material and two layers of cellular thermoplastic material15 and 16 which latter may be less dense than elements 13 and 14. Theprocess shown herein illustrates that in one preferred form, the finallaminated structure comprises a plurality of elements wherein the coremember is a plurality of cellular thermoplastic elements which may be ofdifferent densities. The core member is bounded by exterior laminae orskins which may be of the same material as the core or may bedissimilar, but in any event have a density higher than the core layers.

It is necessary in this invention to provide a laminate which isthermoplastic and which will, upon application of heat thereto, undergodeformation. The deformation characteristics of the individual materialwill, of course,

depend upon the densities as well as softening or distortiontemperature.-

FIG. 2 represents a fragmentary cross-section of a typical laminate 18prepared in accordance with the 8 in the thermoforming operation. Hereelements identifled as 13c, 14c, 15c and 16c are compressed to thinnersections and greater densities then the sector designated A.

invention after it passes through the nip of rolls 19 and HO. 3 isintended to depict the variety of forms 20. The following tabledescribes typical examples of which can be made using the process andproduct of laminates prepared in accordance with this invention. theinstant invention. it will be noted that the selective TABLE OF EXAMPLESExample I II III IV v VI VII VIII IX ABS ABS ABS PVC APVC PC 60 50 so 1540 150 ABS ABS ABS PVC PVC PVC Thickness 40 40 so so 50 so 15 7o 20Cellglar core elements:

' Material PS PS PS PS Ps PS PS PS PP Thickness 100 100 100 100 100 100100 100 200 Density 1s 1s 20 20 20 20 20 1s 4 Materiul PS PS PS PSThickness 100 100 100 100 Denslty (i 0 J 20 (1:

Material. l s PS PS vs 'lhlcknnss 100 100 100 100 1 Density 6 6 U 20Material PS PS PS PS Thlckness 100 100 100 100 F Density 18 18 9 20Material PS Thickness Density. F:

Material Thickness Density Abbreviations: ABS acrylonltrlle, butadiene,styrene copolymer; PS polystyrene; PVC polyvinyl chloride; PPpolypropylene: APVC acrylic] polyvinyl chloride alloy; PC polycarbonate.

The laminate formed by adhesively uniting the various laminae atpressure rolls l8 and 19 is subjected to thermoforming usingconventional thermoforming apparatus, particularly vacuum formingequipment. The laminate is heated to raise the core elements to thedistortion or softening temperature which in the case of polystyrenefoams ranges from about 160 to 295F., preferably 170 to 190F., afterwhich it is subjected to thermoforming operations such as vacuumforming. In the case of polypropylene foam cores, the softening point issomewhat higher, in the range of 212F.

FIG. 3 illustrates a typical molded sheet subjected to a thermoformingoperation, i.e., vacuum forming. In the area designated A, the drawingshows the crosssection of the expanded elements 13a, 14a, 15a and 16a.These cellular foamed sheets, polystyrene in this particular instance,have been expanded in the range of 1.5 to 5, preferably 2 to 3 timestheir original volume. The central cores 15 and 16 were initially about100 mil (0.100 inch) thick and had a density of about pounds per cubicfoot and after expansion (a and 16a) had a density of about 5 pounds percubic foot. The outer cellular cores l3'and 14 were initially about I00mil thick and had a density of about 20 pounds per cubic foot (PCP) andafter expansion had a density of about 10 PCF. Their thickness, ofcourse, increased substantially.

The area of FIG. 3 designated B shows a crosssection of a thermoformedpiece where the mold contours were such that additional expansionoccurred resulting in increased thickness. It is to be noted that coreelements 15b and 16b have expanded substantially more than the coreelements 13b and 14b.

The area designated C in FIG. 3 shows compression localized expansioncapability of the thermoplastic core elements especially in areas suchas B, illustrates that self-reinforcing elements may be incorporatedinto the sheet structure at the same time that it is thermoformed.

The conditions for thermoforming using a vacuum mold have been discussedabove. The laminate of Example l was used to form a camper top employinga Comet Rotary Machine. The heating conditions were 23 percent of inputon the top heater and 57 percent of input on the bottom heater for 105seconds. Preheating was used with the laminate of Examples III to V".With Example V the preheat oven heaters were operated at 9 and 18percent of full input for 255 seconds. The final heat oven was operatedat input rates of 17 percent (top heater) and 34 percent (bottom) for255 seconds. A Comet four stage Rotary Machine with two stage heatingwas used in these examples.

The laminates of the present invention are unexpectedly superior tothose known in the art. For example. a laminate prepared in accordancewith the present invention having skin elements of 60 mil ABS sheet anda core member comprised of two outer core sheets of mil polystyrene foamhaving a density of 18 PCF and three inner core sheets of milpolystyrene foam having a density of 5 PCF.

This laminate was compared with a conventional rigid polyurethane foamlaminate having a 5: inch overall thick polyurethane core with a densityof 6 PCF having been foamed in situ between 60 mil ABS sheeting. Thelaminate of this invention had a flexural fail point on beam loading,14.5 inch span with a center load of 192 pounds at 1.75 inch deflectionas compared to the urethane laminate which had a fail point of 156pounds at a 1.9 inch deflection, which give fail load/deflection ratiosof 82 and 109 respectively. A comparison of impact resistance alsoshowed unexpected superiority. Thus the laminate of this invention hadan impact strength (using a k inch falling dart ram) of 16 foot poundsto rupture compared with 6 foot pounds for the above described urethanefoam laminate.

In the embodiments of this invention which comprise a laminate asdescribed in Examples I, ll, Ill and V, and illustrated in FIGS. 2 and 6of the drawing, it has been found that it is possible to utilize lowcost, very low initial density central core members which are highlyresponsive to heat during thermoforrning and have the capacity ofsubstantial expansion, if these low density central core elements aresurrounded by more heat resistant laminae adjacent the skin elements.These intermediate laminae have heat buffering properties in that theyimpede the transfer of heat from the skins to the innermost core to theend that the skins are at the proper thermoforming temperature but thelow density innermost cores are protected from undue transfer of heatand excessive temperatures which could result in collapse of the cellstructure. The structure referred to in the preceeding is characterizedby thinner skin portions and a lower overall core density whilemaintaining greater strength properties than could be achieved if coreelements of uniform density or strength were used. This obviouslyprovides a more economic product because of the relatively superiorratio of performance to weight.

In FIGS. 4, and 6, there are illustrated in fragmentary enlargedcross-section, various embodiments of the invention such as are shown inthe drawings. The cores are designated by the letters A through F suchas is the designation of the various examples.

What is claimed is:

1. An essentially rigid thermoformable structural laminate ofthermoplastic polymeric material in sheet form and suitable fortherrnoforming to finished goods, said laminate comprising:

a. a core member formed from a plurality of closed cell thermoplastic,extruded, partially expanded and relatively thin foam sheet elementsadhesively united together with a heat deformable, elastomeric adhesive,

b. the foam sheet core-elements being made from a polymeric materialselected from the group consisting of polystyrene, polypropylene,polyethylene and copolymers of styrene, butadiene and acrylonitrile,

c. A pair of exterior skin sheets of non-cellulsr thermoplasticpolymeric material selected from the group consisting of, polyvinylchloride, acrylic polymers, copolymers of acrylonitrile, butadiene andstyrene (ABS), polycarbonates, polystyrene copolymers andacrylate-methacrylate copolymers, the material being of normal densityadhesively united to the surface of the core member with a heatdeformable elastomeric adhesive,

d. the laminate having a first state and configuration wherein the coresheets are parallel thin sheets of uniform thickness, and

e. the laminate having a second state and configuration afterapplication of heat in a subsequent thermoforming operation wherein theexterior skin sheets retain their original thickness but the foam sheetelements expand and produce portions which are substantially thickerthan in the first state,

said laminate being further characterized as maintaining its structuralintegrity as a laminate while each of the adhesively united laminaemaintain their individual structural integrity under load stress and arecapable of lateral movement with respect to each other duringthermoforming.

2. A laminate according to claim 1 wherein the skin member is anon-cellular thermoplastic sheet having a density of from about 60 topounds per cubic foot.

3. A laminate according to claim 1 wherein the skin member has athickness of from about 5 to mils.

4. A laminate according to claim 1 wherein the skin member ispolyvinylchloride resin.

5. A laminate according to claim 1 wherein the cellular core elementshave a density of from about 3 to about 30 pounds per cubic foot.

6. A laminate according to claim 1 wherein the core comprises aplurality of elements each having a thickness of from about 30 to 150mils.

7. A laminate according to claim 1 wherein the skin member is anon-cellular thermoplastic sheet having a density of from about 60 to 90pounds per cubic foot and a thickness of from about 5 to 150 mils.

8. A laminate according to claim 1 wherein the core elements arearranged with the least dense core element or elements in the centralportion of the core member.

' UNI ED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,755,063 Dated August 28, 1973 InVentOI-(S') David H. Massey and RobertB. Anderson It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

.Col. 1, l. 5 "abondoned" should be "abandoned" Col. 1, l. 45' After"plastic" insert boat Col. 6, l. 9. "sheet" should be "sheets" Col. 7,l. 4 "the" should be "this" Col. 10, l. 3 I

Claim 1 Y "non-cellulsr" should be "non-cellular" Signed and sealed this19th day of March lQZL (SEAL) Attest:

EDWARD M.FLETCHER, JR. C. MARSHALL DANN Attesting Officer 7 Commissionerof Patents i FORM V uscoMM-oc scan-Poo i U!- GOVIINHINT PRINTING ol'HCI:I... O-IC-J!

2. A laminate according to claim 1 wherein the skin member is anon-cellular thermoplastic sheet having a density of from about 60 to 90pounds per cubic foot.
 3. A laminate according to claim 1 wherein theskin member has a thickness of from about 5 to 150 mils.
 4. A laminateaccording to claim 1 wherein the skin member is polyvinylchloride resin.5. A laminate according to claim 1 wherein the cellular core elementshave a density of from about 3 to about 30 pounds per cubic foot.
 6. Alaminate aCcording to claim 1 wherein the core comprises a plurality ofelements each having a thickness of from about 30 to 150 mils.
 7. Alaminate according to claim 1 wherein the skin member is a non-cellularthermoplastic sheet having a density of from about 60 to 90 pounds percubic foot and a thickness of from about 5 to 150 mils.
 8. A laminateaccording to claim 1 wherein the core elements are arranged with theleast dense core element or elements in the central portion of the coremember.